A uniform or homogeneous magnetic field is a keystone criterion of magnetic resonance systems. In theory a uniform current density over the surface of a sphere will produce a perfectly uniform magnetic field within the sphere. However, spheres are not used in magnetic resonance systems, instead solenoids are used for generating the main static magnetic field that must be uniform or homogeneous. Solenoids are used because they are easily suspended as a single unit in a cryogenic environment. The uniformity of the magnetic field generated by the solenoid increases as the solenoid increases in length relative to its diameter. Practical considerations, however, limit the length of the superconducting magnets, i.e. the solenoids used in magnetic resonance imaging systems. In general the superconducting magnets that have an outside diameter of 2 meters and are 2 meters in length can be adjusted to have a field uniformity of 25 parts per million or better over the 50 cm. diameter imaging region.
The necessity for homogeneous fields can easily be understood when, for example, one considers that the difference in resonance frequencies of different chemicals is something in the order of about 10 parts per million. In hydrogen magnetic resonance imaging it is known that the difference between hydrogen in water molecules and hydrogen in fat molecules is in the order of 3 parts per million. It is readily apparent that the more homogeneous the static magnetic field is the more accurate is the information that is obtained by the magnetic resonance system.
There are two types of systems used for improving the homogeneity of the magnetic fields generated by the magnetic resonance equipment. They are commonly referred to as: passive shimming and active shimming.
Active shimming comprises the use of a plurality of additional coils strategically placed within the magnet so as to improve the homogeneity of the main static magnetic field when these additional supplementary or active shimming coils are energized. The use of active shimming coils provides benefits and detriments. The benefits include the fact that it is relatively easy to change the effects of the shimming coil by varying the currents individually in each of the shimming coils. However, the shimming coils tend to counteract each other. Thus when the current in a shimming coil is varied to correct an inhomogeneity in one section of the magnetic field an inhomogeneity may be generated in another section (or another degree of the magnetisim). Hence, variations may be required in the current input to selected ones of the other shimming coils. Therefore, time consuming iterative variations in the shimming coil currents are necessary to obtain the desired magnetic field uniformity.
Passive shimming is accomplished by attaching magnetizable metallic sections within the magnet so as to vary the magnetic field distribution in efforts to improve the homogeneity. With relatively small nuclear resonance spectrographic equipment, shimming is relatively easy since the magnets in such equipment are relatively small, and therefore; it is easier to measure the field and easier to vary the field either passively or actively. However, with the onset of the use of magnetic resonance imaging systems, shimming either passively or actively became much more difficult since, among other things, the size of the magnetic field which must be homogeneous is greatly enlarged. Due to the relative ease of varying the homogeneity of the field using shim coils a lot of the inhomogeneity correction is done with active shimming.
Relatively recently passive shimming has been aided by software programs that have been devised. The programs use data from measurements of the homogeneity of the field to indicate where "iron" (shims) has to be placed to improve the homogeneity of the field. By iteratively, strategically placing the shims as indicated by the program eventually the field homogeneity is brought to the desired standard. However, because of the large number of repetitive shim placement steps required passive shimming remains extremely time consuming even with the use of the computerized algorithms for indicating where the shims should be placed to correct the field inhomogeneity. The passive shimming described herein can shim at least ten harmonics in the axial direction compared to five using prior art active shimming.
One of the reasons that passive shimming is so time consuming is that up to now there has been no efficient method of holding and locating the shims in accordance with the computerized indications of shim locations. Accordingly, it is an object of the present invention to provide means for denoting shim locations that can be coordinated with the shimming programs and for removably retaining the shims in the places indicated by shimming programs.